Added delete key to level editor select mode.
Added SaveData class and factored out that data from the screen class.
Factored out screen class's level-specific stuff into a LevelData member.  Since the level browser depends on hijacking the screen class to do its dirty work, the browser is now temporarily broken.
Moved level browser stuff from picker.cpp to level_picker.cpp.
Added license header to the newer files.
Replaced picker.cpp global team state with the one stored in the screen class.  I need to look over the autosaving to make sure it is consistent.
Fixed up some bugs introduced by the switch from global state.  There is still a memory bug lurking...